1. Field of the Invention
The present invention relates to a radiation detection apparatus and a detection system that are applied to medical image diagnostic apparatuses, nondestructive inspection apparatuses, analytical apparatuses that use radiation, and the like.
2. Description of the Related Art
A technique for manufacturing thin-film semiconductors is utilized in the manufacture of a detection apparatus including an array of pixels (pixel array). Notably, in a radiation detection apparatus, switch elements such as thin-film transistors (TFTs) and conversion elements such as photoelectric conversion elements are combined. In the related art, it is known, for example, that a substrate having a size of 43×43 cm on which pixels are arranged in an array at a pitch of 150 to 200 μm has been used to manufacture a radiation detection apparatus. In a conventional radiation detection apparatus, a driving circuit drives the pixel array in units of rows through driving wires, and a read circuit outputs signals from the pixel array driven in units of rows parallel to one another through signal wires as serial signals. The driving and read circuits are typically prepared in an integrated circuit composed of monocrystalline silicon, and subsequently mounted onto the substrate using chip-on-glass (CoG) technology to configure the detection apparatus.
In such a detection apparatus, currently, the pitch of the pixels is desired to be smaller while the size of the substrate in the related art remains the same. More specifically, a pixel array having a pitch of 50 to 80 μm is expected. Therefore, it would be difficult to mount, using chip-on-glass technology, the driving circuit prepared in the integrated circuit composed of monocrystalline silicon on the substrate on which the pixel array is provided. In addition to the reduction of the pitch of the pixels, a system-on-panel technology is expected in which the driving circuit and the like are formed integrally on the substrate as a unit using a TFT process, in order to reduce the number of components used, and to increase the area of the substrate occupied by the pixel array.
On the other hand, in a liquid crystal display apparatus using TFTs, the system-on-panel technology is being developed in which the driving circuit and the like are formed on the substrate as a unit using a TFT process. J. H. Oh et al, in an article entitled “2.0 inch a-Si:H TFT-LCD with low noise integrated gate driver”, Proceedings of Soc. Info. Disp., 2005, 942-945, discloses a driving circuit in which a plurality of stages of unit circuits are formed on a substrate as a unit using a TFT process and prepared in units of rows of pixels while corresponding to driving wires. Each unit circuit includes an output unit, a first input unit, a second input unit, a third input unit, and a fourth input unit. Here, the first input unit is a portion to which a start signal or an output signal of a unit circuit in the previous stage is input, and the output unit is a portion connected to a driving wire to supply an output signal including conducting voltage and non-conducting voltage of a switch element to the driving wire. The second input unit is a portion to which a clock signal is input, the third input unit is a portion to which the non-conducting voltage of the switch element is input, and the fourth input unit is a portion to which a reset signal or an output signal of a unit circuit in the next stage is input. Each unit circuit also includes a first capacitor, one end of which is connected to the output unit, a first thin-film transistor connected between the first input unit and another end of the first capacitor in series with the first capacitor, and a second thin-film transistor provided between the second input unit and the output unit. Either a source or a drain and a gate of the first thin-film transistor are connected to the first input unit, and the other of the source and the drain of the first thin-film transistor is connected to the other end of the first capacitor at a first node P. A gate of the second thin-film transistor is connected to the first node P, either a source or a drain of the second thin-film transistor is connected to the second input unit, and the other of the source and the drain of the second thin-film transistor is connected to the output unit. Each unit circuit also includes a third thin-film transistor connected between the third input unit and the other end of the first capacitor in series with the first capacitor and a fourth thin-film transistor provided between the third input unit and the output unit. A gate of the third thin-film transistor is connected to a second node Q, either a source or a drain of the third thin-film transistor is connected to the third input unit, and the other of the source and the drain of the third thin-film transistor is connected to the other end of the first capacitor at the first node P. A gate of the fourth thin-film transistor is connected to the second node Q, either a source or a drain of the fourth thin-film transistor is connected to the third input unit, and the other of the source and the drain of the fourth-thin film transistor is connected to the output unit. Each unit circuit also includes a second capacitor provided between the third input unit and the gate of the fourth thin-film transistor.
Furthermore, each unit circuit includes a fifth thin-film transistor provided between the fourth input unit and the gates of the second and fourth thin-film transistors and a sixth thin-film transistor provided parallel to the second capacitor. One end of the second capacitor is connected to the third input unit, and another end of the second capacitor is connected to the gate of the fourth thin-film transistor at the second node Q. Either a source or a drain and a gate of the fifth thin-film transistor are connected to the fourth input unit, and the other of the source and the drain of the fifth thin-film transistor is connected to the second node Q. A gate of the sixth thin-film transistor is connected to the first input unit, either a source or a drain of the sixth thin-film transistor is connected to the third input unit, and the other of the source and the drain of the sixth thin-film transistor is connected to the second node Q. Here, the threshold voltage of each of the first to sixth thin-film transistors is denoted by Vth, the maximum and minimum values of voltage of signals input to the components of the unit circuit other than the third input unit are denoted by VDD and VSS, respectively. The non-conducting voltage supplied to the second input unit is also denoted by VSS. When selected, this unit circuit can execute a so-called bootstrap operation, in which the maximum value of voltage of the clock signal input to the second input unit, namely VDD, is output by the first and second thin-film transistors and the first capacitor. In addition, when this unit circuit has not been selected, the unit circuit can output the non-conducting voltage using the fifth and sixth thin-film transistors and the second capacitor. That is, when the unit circuit has been selected, the first and second thin-film transistors and the first capacitor serve as a circuit that supplies the maximum value of voltage of the clock signal input to the second input unit, namely VDD, to the output unit as the conducting voltage of the switch element of a pixel. When the unit circuit has not been selected, the third to sixth thin-film transistors and the second capacitor serve as a circuit that supplies the non-conducting voltage VSS to the output unit.
Because there is leakage current (channel leakage) in a thin-film transistor, there is a problem in keeping the non-conducting voltage unchanged (constant) when the unit circuit has not been selected. This is because, when there is channel leakage in the fifth and sixth thin-film transistors, the potential of the second node Q gradually decreases and, after an extended period of time elapses, it becomes difficult to supply voltage higher than Vth to the gate of the fourth thin-film transistor. In such a case, because the fourth thin-film transistor becomes non-conductive, the voltage VSS cannot be supplied to the output unit of the unit circuit, thereby causing the output unit of the unit circuit to float. Specifically, as used herein, a circuit is said to “float” when the circuit is not grounded and stays at some potential other than a ground reference potential; that is, the conductor(s) of the circuit are isolated from ground. A floating circuit can have safety issues because there is no low-impedance path to ground. However, this type of circuit can also help isolate a system from interference problems, for example. The second input unit and the output unit directly form capacitive coupling due to gate-to-drain capacitance Cgd or gate-to-source capacitance Cgs and the parasitic capacitance of a region in which a wire that supplies the clock signal and a driving wire intersects. Therefore, when the output unit of the unit circuit floats, variation in the potential of the clock signal input to the second input unit affects the driving wire, thereby causing the potential of the driving wire to vary. The driving wire has parasitic capacitance in the region in which the driving wire intersects with the signal wire, and the variation in the potential of the driving wire affects the potential of the signal wire through the parasitic capacitance, thereby mixing the variation in the potential of the driving wire into a signal output from a pixel through the signal wire. Therefore, a noise component caused by the variation in the potential of the driving wire becomes large in the signal output from the pixel through the signal wire, and accordingly the signal-to-noise ratio (S/N) of a signal obtained from the detection apparatus can undesirably decrease.